Radio spectrum is a very scarce resource due to its limited availability for cellular communications and demand for very high data rate applications, which is growing exponentially. Frequency reuse utilizes the available radio resources efficiently by using same frequency spectrum in multiple cells which are geographically located at different places. Further, the frequency reuse is a preferred mode of deployment where each sector uses entire available frequency resources thus, introducing co-channel interference (CCI) and severely affecting users at a boundary between cells. Also, a greater number of antennas at a receiver can cancel or suppress the interference from co-channels, which in turn increases spectral efficiency.
MIMO is another technology that can help in increasing capacity of a wireless system for a given bandwidth and power; this is the key reason for using the MIMO technique in technologies such as Long-Term Evaluation (LTE), Worldwide Interoperability for Microwave Access (WiMAX), and Wireless Fidelity (Wi-Fi). Further, the MIMO can be classified into two types such as a single-user MIMO (SU-MIMO) and a multi-user MIMO (MU-MIMO). In case of the SU-MIMO, multiplexed data streams belong to same receiver and rank adaptation offers the possibility to dynamically adapt a number of data streams for the receiver to current channel conditions. Further, multiple antennas can also be used to increase the diversity by transmitting multiple copies of same information. At the receiver, by appropriately combining of these replicas, a more reliable reception can be achieved.
In case of third generation (3G) or fourth generation (4G) technologies, as the operating frequency is in hundreds of MHz, independent channel realizations can be obtained with enough spacing between the antennas. Since space is less of a constraint at a base station (BS), deploying the multiple antennas at the BS is simple. However, the deployment of multiple antennas at the receiver can be a challenge due to the small device form factor, cost, and complexity issues. Therefore, the number of spatial dimensions of the SU-MIMO that can be exploited is limited by the number of antennas at the receiver.
To overcome the above limitations, the antennas of the receivers located in different geographic locations can be treated as a part of a larger MIMO, and same time-frequency resources are shared by more than one receiver belonging to multiple users located in different geographical locations. This type of the MU-MIMO improves the overall system throughput by increasing spectral efficiency instead of per receiver peak throughput. Further, the performance of the MU-MIMO mainly depends on the scheduling technique. However, the receivers scheduled for the MU-MIMO still experience multi-user interference when Channel State Information (CSI) is outdated or when users experience non-orthogonal channel between them.
The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.